Transition-metal catalyzed C—C bond modulation via olefin metathesis is of considerable interest and synthetic utility. Initial studies in this area were based on catalytically active mixtures consisting of transition-metal chlorides, oxides or oxychlorides, cocatalysts such as EtAlCl2 or R4Sn, and promoters including O2, EtOH or PhOH. These systems catalyze olefin metathesis reactions, however their catalytic centers are ill-defined and systematic control of their catalytic activity is not possible.
Recent efforts have been directed towards the development of well-defined metathesis active catalysts based on transition metal complexes. The results of research efforts during the past two decades have enabled an in-depth understanding of the olefin metathesis reaction as catalyzed by transition metal complexes.
Group VIII transition metal olefin metathesis catalysts, specifically ruthenium and osmium alkylidene complexes, have been described in U.S. Pat. Nos. 5,312,940 and 5,342,909 and U.S. patent applications Ser. Nos. 08/282,826 and 08/282,827, all of which are incorporated herein by reference. The ruthenium and osmium alkylidene complexes disclosed in these patents and applications are of the general formula:
wherein M is ruthenium or osmium, X and X1 are anionic ligands, and L and L1 are neutral electron donors.
U.S. Pat. Nos. 5,312,940 and 5,342,909 disclose specific vinyl alkylidene ruthenium and osmium complexes and their use in catalyzing the ring opening metathesis polymerization (“ROMP”) of strained olefins. In all of the specific alkylidene complexes disclosed in these patents, R1 is hydrogen and R is either a substituted or unsubstituted vinyl group.
U.S. patent application Ser. Nos. 08/282,826 and 08/282,827 disclose specific vinyl alkylidene ruthenium and osmium complexes and their use in catalyzing a variety of metathesis reactions. The catalysts disclosed in these applications have specific neutral electron donor ligands L and L1; namely, phosphines in which at least one substituent is a secondary-alkyl or cycloalkyl group. In all of the specific alkylidene complexes disclosed in the patent applications, R1 is hydrogen and R is either a substituted or unsubstituted vinyl group.
In another example, U.S. Pat. No. 5,959,170, issued Oct. 19, 1999, discloses similar ruthenium or osmium catalysts, wherein L and L1 are each trialkyl phosphine ligands, where at least one of the alkyl groups on the phosphine is a secondary alkyl or cycloalkyl. The catalysts are suitable for catalyzing an array of metathesis reactions including ring-opening metathesis polymerization of cyclic olefins, ring closing metathesis of acyclic dienes, cross metathesis involving at least one acyclic or unstrained cyclic olefin, depolymerization of unsaturated polymers and synthesis of telechetic polymers.
In another example, U.S. Pat. No. 6,111,121, issued Aug. 29, 2000, reported novel ruthenium alkylidene compounds that are stable in the presence of a variety of functional groups and can be used to catalyze olefin metathesis reactions on unstrained cyclic and acyclic olefins. The alkylidene compounds disclosed comprise a compound of Formula I, wherein M is Os or Ru; R1 is hydrogen; R is selected from the group consisting of hydrogen, substituted or unsubstituted alkyl, and substituted or unsubstituted aryl; X and X1 are independently selected from any anionic ligand; and L and L1 are independently selected from any neutral electron donor. The compounds are suitable for use in ROMP and ring closing metathesis reactions.
In further examples, U.S. Pat. Nos. 6,121,473, and 6,346,652, issued Sep. 19, 2000 and Feb. 12, 2002 respectively, disclose compositions and methods generally involving molybdenum or tungsten-based catalysts of the general formula (II):
wherein K is preferably Mo or W, R1 and R2 each independently selected from C1–C12 alkyl, heteroalkyl, aryl, heteroaryl and adamantyl, and the linked oxygen atoms pertaining to a chiral dialkoxide. The compositions are suitable for use as catalysts for ring-closing metathesis reactions involving racemic dienes, in which enantiomeric cyclic olefin products are generated. Methods are also provided for catalytic enantioselective desymmetrization.
The teachings of U.S. Pat. Nos. 5,959,170, 6,111,121, 6,121,473, and 6,346,652 are also incorporated herein by reference.
In a final example, International Patent publication WO00/71554, published Nov. 30, 2000 discloses ruthenium or osmium based metathesis catalysts that include an alkylidene group and an imidazolidine-based ligand. The inclusion of the imidazolidine ligand to the previously described ruthenium or osmium catalysts provides some improvements to the properties of these complexes. The imidazolidine ligand maintains the functional group tolerance of the previously described ruthenium complexes, and enhances metathesis activity.
Over the past decade, olefin metathesis has emerged as a powerful synthetic tool. For example, ring-closing metathesis (RCM) reactions and cross-metathesis reactions have had a major impact in organic synthesis, including natural products synthesis, (Ref. 1a–1c) while metathesis polymerization techniques have given access to a new class of polymer materials (Ref. 1d). Grubbs' development of robust, functional group-tolerant catalysts based on ruthenium (Ref. 1a) was a milestone in the still ongoing evolution of this methodology (see compound of Formula Ia, FIG. 1). Considerable current effort is directed at discovery of new Ru catalysts with expanded activity, selectivity, and lifetime (an important recent addition being highly reactive N-heterocyclic carbene (NHC) derivatives; Ref. 2–4, 5a, for example see compounds of Formula 1b–1e, as shown in FIG. 1). While exchange of the neutral “L-donor” ligands in these systems is now facile, Ref. 2,5 the steric and electronic consequences are frequently dramatic. Fine tuning of activity and selectivity has remained elusive. In contrast, the Group 6 catalysts pioneered by Schrock afford exceptional control over activity and selectivity, including asymmetric ring-closing and ring-opening metathesis. (Ref. 1c, 6–8) Key to this versatility is the presence of modular, tunable aryloxide or alkoxide ligands, vs. the simple chloride ligands ubiquitous (Ref. 9–12) in the Ru chemistry.
While much effort has focused on modification of neutral “L-donor” ligands in the ruthenium systems (as noted above), modification of the anionic ligands is much less explored, and has so far met with limited success. Metathesis catalysts of low to moderate activity are obtained by replacing chloride with carboxylate (Ref. 9a, b, c): or by the use of heterobifunctional salicylaldimine (Ref. 9d), or NHC-naphtholate ligands (Ref. 10). Installation of alkoxide ligands affords four-coordinate species 2a/b (see FIG. 1, Scheme 1), (Ref. 11,12). Despite the nominal coordinative unsaturation of these complexes, they exhibit near-zero metathesis activity (Ref. 12; Table 1). Reaction of compound 1a or RuCl2(PiPr3)2(CHPh) with phenoxide anion gives alkylidynes (compounds 4a and 4b), via deprotonation of the benzylidene ligand. (Ref. 11)
There remains a continuing need to develop improved catalysts for olefin metathesis reactions, including, but not limited to, ring-opening metathesis polymerization, ring closing metathesis, cross-metathesis reactions (for example involving at least one acyclic or unstrained cyclic olefin), and depolymerization of olefin polymers.